From the past, carbon black and silica have widely been used as rubber-reinforcing fillers and, in general, a Banbury mixer, an open roll, a kneader and other kneading machines have been used extensively to incorporate the fillers into rubber by the dry process (also called the dry blending process).
In recent years, a silica-filled diene rubber composition has been found not only to be more freely colored, lower in environmental pollution and more excellent in tearing strength but also to enable compatibility of both fuel efficiency and high gripping property as compared with a carbon black-filled rubber composition, thus catching public attention as a rubber material for use in tire treads.
However, silica, the surface of which is covered with silanol groups, is strong in auto-agglutination but poor in affinity with diene rubber, thus making it difficult to attain a favorable dispersion into the rubber. A problem was that cross-linked rubber obtained by cross-linking a diene rubber composition filled with silica by the dry process was poor in reinforcing property such as tensile strength and wear resistance as compared with a cross-linked rubber obtained by cross-linking a carbon black-filled rubber composition.
In order to improve affinity of silica to the diene rubber, therefore, proposed are a method for incorporating the composition with a silane coupling agent according to the dry process (Reference 1 referred to) and a method wherein the composition is incorporated with a special rubber into which a functional group having affinity to silica has been introduced according to the dry process (Reference 2 referred to).
However, the composition obtained by the above methods was insufficient in improving affinity of the diene rubber to silica and not satisfactory in improving effect of the reinforcing property with the use of silica.
In general, there is a method for measuring a quantity of rubber content insoluble in toluene, i.e. a solvent giving a favorable solubility (hereinafter referred to as bound rubber) as an index for judging the affinity of the rubber with silica. It was found, however, that the above composition was very small in a quantity of such bound rubber, due to a low affinity of rubber with silica and other reasons.
As a method for increasing a quantity of the above bound rubber in the dry process, there is proposed the use of a special rubber obtained by copolymerization of three special monomers is followed by mixing with silica, thereby providing a rubber composition rich in the bound rubber (Reference 3 referred to).
However, the special rubber used in the above rubber composition is low in molecular weight distribution in terms of the ratio of weight-average molecular weight to number-average molecular weight (hereinafter also referred to as Mw/Mn) as ranging from 1.0 to less than 1.1, due to the process for production employed, and there is a room for improvement in processability and versatility.
In contrast to the dry process, there is also proposed a method for mixing an aqueous dispersion of silica with rubber latex to coagulate simultaneously silica and rubber particles in the dispersion, thereby obtaining a uniform coagulation product (so called co-coagulation).
Also publicly known are, for example, a rubber composition obtained by a method wherein an aqueous dispersion of silica having a mean particle size of 1 μm or smaller treated with a large quantity of a cationic polymer is mixed with rubber latex and a salt is added thereto to effect co-coagulation of the rubber with the silica (Reference 4 referred to) and a rubber composition obtained by a method wherein an aqueous dispersion of silica treated with a silane coupling agent is mixed with rubber latex and an acid is added thereto to effect co-coagulation of the rubber with the silica (Reference 5 referred to). These references did not refer to the amount of the bound rubber in the rubber composition.
The inventors have confirmed, however, that a relative content of the bound rubber to silica is extremely high or at least 1.1 g per gram of the silica in the rubber composition obtained according to the above methods. Thus, there may be possible problems that a rubber composition rich in the bound rubber improves, only to a small extent, the fuel efficiency and gripping property which are merits in using silica in cross-linked rubber produced by cross-linking of the composition, and the rubber composition itself is rendered excessively rigid, easily resulting in the formation of a gel-like product which is unbreakable during kneading. Thus, some room remains for improving processability in the course of kneading.
There is further proposed a method in which an aqueous dispersion of silica is mixed with rubber latex and an organic ionic compound is then added thereto to effect co-coagulation of rubber with silica (Reference 6 referred to). As is apparent from the subsequently-described Comparative Examples for tracing the above methods, it has been found that co-coagulation products obtained by these methods are unable to provide a sufficient affinity of rubber with silica and result in a smaller production of the bound rubber.
There is still further proposed a method in which rubber latex is mixed with an aqueous suspension of silica containing acrylamide/dimethylaminoethyl methacrylate copolymer to effect co-coagulation of rubber with silica (Reference 7 referred to). This reference also fails to describe a quantity of the bound rubber and it was difficult to produce the bound rubber in a sufficient quantity in case a common flocculating agent, acrylamide/dimethylaminoethyl methacrylate copolymer, was used.
Among the rubber compositions wherein silica has been incorporated into a diene rubber having a wide distribution of molecular weight such as SBR with versatile applications, a rubber composition containing the bound rubber in so moderate amount as to exhibit favorable physical properties has not yet been proposed hitherto, as described above.    (Reference 1) Japanese Published Unexamined Patent Application No. H3-252431    (Reference 2) Japanese Published Unexamined Patent Application No. S63-2886    (Reference 3) Japanese Published Unexamined Patent Application No. H7-118449    (Reference 4) Japanese Published Unexamined Patent Application No. 2001-213971    (Reference 5) Japanese Published Unexamined Patent Application No. H10-231381    (Reference 6) U.S. Pat. No. 3,122,518    (Reference 7) U.S. Pat. No. 4,366,285